In a typical electrophotographic image reproduction process, first a latent charge image is formed on a pre-charged photosensitive member by image-wise exposure to light using a light emitting unit, e.g. a unit with a plurality of LED array ICs. This latent image is subsequently made visible on the photosensitive member with charged toner particles. Examples of a photosensitive member are a photoconductive drum or belt. The developed image is transferred directly or via one or more intermediate transfer members to a substrate, where it may be fixed simultaneously or subsequently.
In conventional electrophotographic systems often use is made of light emitting units including LED array ICs, to generate the light, combined with an optical system to focus the light on the photosensitive member. As e.g. disclosed in EP 629 507 and EP 2 036 734 in the name of the Applicant, a LED array is typically composed of a number of LED array ICs arranged on a carrier and connected, e.g. by means of wire bonding, to adjacently attached driver ICs. Usually the carrier also acts as a heat sink. The light generated by the light-emitting diodes, LEDs, is accurately focused on the photosensitive member by means of a lens block positioned between the LEDs and the photosensitive member.
Due to positioning errors and tolerances when mounting the LED arrays on the carrier, the length of the light emitting area of different light emitting units may be different. When light emitting units with different lengths are used in the same printing apparatus the image quality will be significantly decreased. To deal with such differences it is known to classify manufactured light emitting units in length classes. However, it can be easily understood that this creates problems with stock, exchangeability of the units, production requirements, etc.